When viewed worldwide, the number of hard-of-hearing persons is supposed to be increasing. Hearing loss is categorized into conductive hearing loss, sensorineural hearing loss and mixed hearing loss that includes both thereof, depending on a part of disorder in the auditory system. A character given as common to these categories of hearing loss is incapability of or difficulty in sensing a certain frequency band of sounds that a hard-of-hearing person wants to hear.
The miniaturization and speed-up of digital signal processing processors (DSP) in recent years have enhanced freedom in designing hearing aids, and thus have caused the mainstream of the hearing aids to shift from analog to digital.
Currently, a method commonly used in audio signal processing for digital hearing aids is a multi-channel method using the conventional signal processing theory. In this method, first, an analog audio signal that comes in from a microphone is passed through a microphone amplifier, and then is subjected to an AD-conversion process.
Secondly, a digital signal resulting from the AD conversion is first split into a multiple frequency bands by bandpass filters, and then at each frequency band the signal is processed via a digital filter with a gain being increased or decreased according to the auditory characteristics of a hard-of-hearing person.
The divided frequency bands are then resynthesized.
And finally, the signal undergoes a DA-conversion process and then is outputted as an analog audio signal from a speaker by way of a speaker amplifier.
In relation to this, references related to the present invention include patent literature 1, 2 as cited below. The patent literature 1, 2 below are attributable to the present inventor(s), where the patent literature 1 discloses a method for designing a digital filter according to sampled-data H∞ control theory, whereas the patent literature 2 discloses a sampling rate conversion device designed according to sampled-data H∞ control theory.
[Citation List]
[Patent Literature]
Patent Literature 1                Japanese Patent Gazette No. 3,820,331        
Patent Literature 2                Japanese Patent Gazette No. 3,851,757        